# 3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

^{8}m

^{3}; among them, the channel below 90 m elevation accounts for 73% of the total siltation. The average deposition thickness in the deep channel before the dam is 33.5 m, with a maximum deposition thickness of 63.7 m [15]. Of all the deposits, 0.47 × 10

^{8}m

^{3}was deposited during the period of 2008–2013, with a maximum thickness of 16.8 m [3]. Until recently, the elevation of the deposits was lower than that of the power plant inlets [16].

^{3}[19]. Particles deposited in the dam area are so fine that their initial dry bulk density is small, and they are able to move as a fluid. Previous simulations of the sedimentation pattern in the dam area of the TGR did not consider the fluid characteristics of the deposits, and their predictions were significantly different from the observations [11,12,20]. The differences indicate the significance of considering the fluid characteristics of the deposits. By coupling a 1D gravity-driven flow model, which depicts the motion of sediment deposits, with a traditional 1D flow and sediment transport model, the accumulation processes of fine-grained sediments in the dam area of the TGR had been preliminarily identified [21,22]. However, more work is necessary to determine the mechanisms of the fine-grained sedimentation pattern in large reservoirs.

## 2. Materials and Methods

#### 2.1. Features of Fine-Grained Sediment Deposition in Deep Reservoirs

^{3}. The TGR has been in operation since June 2003. The construction of large hydraulic projects such as the TGD has generated problems in navigation, environment, and ecology, all of which are related to sediment transport processes, including advection, diffusion, deposition, and erosion.

**Figure 1.**Location of the TGD on the Yangtze River. (The left part of the figure is based on [23]).

^{3}) in the form of sludge are called fluid mud. The fluid mud has significant fluidity and can flow or deform under its own weight. If the surface slope exceeds a certain value, the fluid mud will move down the slope. This may be the main reason that most of the deposits were found in the deep channel of the TGR and that the surface of the sludge is horizontal.

#### 2.2. Numerical Simulation Method for Transport and Settlement of Fine-Grained Sediment

#### 2.3. Numerical Simulation Method for Gravity-Driven Motion of Fine-Grained Sediment Deposits

^{−9}; and $\beta $ = 16.719 based on previous research [25].

#### 2.4. Numerical Simulation Procedure

## 3. Results

#### 3.1. Flow Field and Suspended Sediment Concentration

^{3}/s and a pool level of 135 m above sea level. Strong 3-D flow features can be observed in the bends near the dam area, and secondary currents are significant. The velocity vectors near the bottom point to the convex bank, whereas the surface vectors point to the concave bank. Based on the calculated flow velocities, the suspended load concentration can be calculated by solving the convection–diffusion equation. Figure 8 shows the calculated suspended load concentrations on typical cross-sections. The sediment concentration decreases much more rapidly in wider valley areas. Due to the impact of the secondary currents shown in Figure 7, the maximum concentration occurs at the convex side, whereas the minimum occurs at the concave side. Figure 9 shows a comparison of the observed and simulated velocities and suspended load concentrations on typical cross-sections (see Figure 2 for the locations of the sections) for a flow discharge of 24,600 m

^{3}/s. In general, the 3D numerical model accurately predicted the flow patterns and suspended load concentrations in the dam area of the TGR.

#### 3.2. Features of Sedimentation Pattern

## 4. Discussion

#### 4.1. Difference between Calculation and Measurement

#### 4.2. Shortcomings of Present Method

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 4.**Schematic processes of fine-grained particle sedimentation in the dam area [20].

**Figure 7.**Calculated velocity fields near the bottom and the water surface (the blue lines are the velocity vectors close to the water surface, and the black lines are the vectors close to the bottom).

**Figure 8.**Simulated suspended load concentrations on typical cross-sections. (

**a**) the discharge is 6300 m

^{3}/s and water level is 139 m; (

**b**) the discharge is 32,100 m

^{3}/s and water level is 135 m.

**Figure 9.**Verification of the velocity and sediment concentration distributions on typical cross-sections. (

**a**) on section S30+1; (

**b**) on section S34.

**Figure 12.**Comparisons of the sediment distributions on a typical cross-section between the observations and simulated results. (

**a**) on section S30+1; (

**b**) on section S32; (

**c**) on section S34; (

**d**) on section S37.

**Figure 14.**Computed velocity distribution in front of the dam (the discharge is 35,600 m

^{3}/s and water level is 135.1 m).

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**MDPI and ACS Style**

Jia, D.; Zhou, J.; Shao, X.; Zhang, X.
3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs. *Water* **2021**, *13*, 1868.
https://doi.org/10.3390/w13131868

**AMA Style**

Jia D, Zhou J, Shao X, Zhang X.
3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs. *Water*. 2021; 13(13):1868.
https://doi.org/10.3390/w13131868

**Chicago/Turabian Style**

Jia, Dongdong, Jianyin Zhou, Xuejun Shao, and Xingnong Zhang.
2021. "3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs" *Water* 13, no. 13: 1868.
https://doi.org/10.3390/w13131868